The processes known hitherto operate on the principle that in so called deep drawing apparatus, i.e. thermoforming machines, films, such as plastics films, are heated primarily by infrared radiation or by so called contact heating and in this way are brought into a plastic state.
The plastic behaviour of these films is used to change this plasticized plastics film into the desired shape using a rotating vacuum roller or a special deforming tool (upper and lower deforming plate), by means of compressed air and optional prestretching dies integrated in the deforming tool.
In both processes, relatively large amounts of heat are needed to heat the plastics film in order to bring them into the plastic state.
In order to keep the plasticized plastics film, which is in a certain form in the moulding tool, in a stable shape and to allow it to be removed from the mould, this tool must be kept at a temperature at which the plasticized plastics film solidifies, so that this deformed plastics film retains a stable shape.
The heating energy which is required in the processes described is not inconsiderable. At the same time further energy is required to cool the film in order to make the deformed film stable in shape again.
In the packaging industry very large quantities of pvc films and combinations thereof are processed using these methods. However, these pvc films, the waste material stamped out and empty packages are environmentally harmful, if not toxic and cannot readily be disposed of.
Film manufacturers have now started to produce films from polypropylene (pp) which are environmentally more acceptable and can in the end be burned to produce water vapour and carbon dioxide without forming any acids or harmful gases.
The disadvantages of these polypropylene films is that they have a poorer heat transmission that pvc films. This means that more energy is required to process these films by the known methods. At the same time this means that more energy is required to cool the deforming tools as well.
Examples from Japan and the USA, where the polypropylene films have been processed by the methods described for some years, are proof of this.
Another disadvantage of polypropylene films is that existing deep drawing tools or apparatus, which were originally designed for processing pvc films, for the most part cannot be used for deforming polypropylene films since these systems are unable to accommodate the heating and cooling equipment needed. Nor can the existing tools for processing pvc films be used for the more difficult processing of polypropylene films since the expansion coefficient or the shrinkage factor of these two types of plastics films are very different.
German Patent 3203336 describes an apparatus of the type mentioned hereinbefore by means of which a step is formed on a portion of a cup-shaped container. The step is formed by means of a forming die acted upon by ultrasound, with the film being sharply round.